Research and Development Report
July 1957
Report 1153 Lab.v.
Scheepsbouwkunde7
Technische Hogeschool
Delft
NAVY DEPARTMENT
THE DAVID W. TAYLOR MODEL BASIN
WASHINGTON 7. D.C.A COMPARISON OF PITCH MEASUREMENTS ON AN OSCILLATING
TOWED BODY USING A VERTICAL GYRO AND A PENDULUM INDICATOR
by
A COMPARISON OF PITCH MEASUREMENTS ON AN OSCILLATING TOWED BODY USING A VERTICAL GYRO AND A
PENDULUM INDICATOR by
Samuel M. Y. Lum and Chester 0. Walton
TABLE OF CONTENTS Page ABSTRACT INTRODUCTION 1 EXPERIMENTAL SET-UP DISCUSSION OF RESULTS BASIN TESTS DYNAMIC CALIBRATION
CONCLUSIONS AND RECOMMENDATIONS
ACKNOWLEDGMENTS 8
APPENDIX A. - THE DAVID TAYLOR MODEL BASIN HEAVING FACILITY
APPENDIX B. - ANALYSIS OF THE EFFECTS OF TOWED BODY MOTION 11
ON A PENDULUM ANGLE INDICATOR
4
7
Figure 4b Figure 4e Figure 4d Figure
5
-Figure 6a Figure 6b Figure 6c Figure 6d Figure 7 -LIST OF ILLUSTRATIONSFigure I - Towed Body Used in Experiment Figure 2 - The Heaving Towpoint Facility
Figure
3 -
Experimental Set-Up Showing Dynamic Calibration ofGyro and Pendulum Indicator
Figure 4a - Sample Records Showing Pitch Response of Towed Body as Indicated by Gyro and by Edcliff Pendulum, w = 0
Radian per Second
Sample Records Showing Pitch Response of Towed Body as Indicated by Gyro and by Edcliff Pendulum, w = 2.0
Radians per Second
Sample Records Showing Pitch Response of Towed Body as Indicated by Gyro and by Edcliff Pendulum, w - 2.4
Radians per Second
Sample Records Showing Pitch Response of Towed Body as Indicated by Gyro and by Edcliff Pendulum, m = 2.8
Radians per Second
Comparison of Maximum Pitch Amplitudes of Towed Body as Measured by Gyro and Pendulum for Different Input
Frequencies
Sanborn Traces of Gyro and Pendulum Dynamic Calibration,
w = 2.02, 2.17 Radians per Second
Sanborn Traces of Gyro and Pendulum Dynamic Calibration,
w = 2.26, 2.38 Radians per Second
Sanborn Traces of Gyro and Pendulum Dynamic Calibration,
w = 2.49, 2.57 Radians per Second
Sanborn Traces of Gyro and Pendulum Dynamic Calibration,
U) = 2.81,
2.88
Radians per SecondEffect of Horizontal Acceleration and Frequency on Gyro
and Pendulum Output
-A COMP-ARISON OF PITCH ME-ASUREMENTS ON -AN OSCILL-ATING TOWED BODY USING A VERTICAL GYRO AND A
PENDULUM INDICATOR by
Samuel M. Y. turn and Chester 0. Walton
ABSTRACT
A comparison is made of the relative accuracy of a pendulum indicator as compared with a vertical gyro in determining the
pitching motion of a cable-towed body under forced oscillation.
It is shown that serious errors are introduced in the measurement of body attitude with a pendulum indicator when the body is
sub-jected to accelerating forces.
INTRODUCTION
In the determination of the pitching response of a cable-towed body subjected to an oscillating input, pendulum indicators
are frequently used to measure angular orientation of the body.
Generally, space availability, the number of conductors required, simplicity of operation and recording, and cost economy have
dictated the use of pendulums. Since, however, these devices
were designed only for static measurements, the question of their
accurac under dynamic conditions has been cause for serious
con-cern. Recently, during the course of obtaining motion data on a
cable-towed body, the Model Basin had occasion to compare a
pendulum indicator with a vertical gyro under dynamic conditions. This report presents the results of this comparison.
EXPERIMENTAL SET-UP
The specifications for the gyro and pendulum units are given
in Table 1. The towed-body in which they were installed is shown in Figure 1. This body was constructed for another purpose and
was used for these tests merely as a matter of convenience. As
a result, no particulars need be given except that the body was
21
feet long and contained internal ballast to provide metacentricstability and permit trimming to zero in the submerged condition. The gyro and pendulum were located as shown in the following sketch.
The tow cable besides being a strength member provided sufficient electrical conductors for powering the gyro and
pendulum and for transmitting the output signals to a 2-channel
pen recorder. The cable was connected to the TMB heaving towpoint facility shown in Figure 2 and described in detail in Appendix
A.
Basin tests consisted of simultaneously recording the out-puts of the gyro and pendulum with the body towed at various
speeds and subjected to an oscillating input at the towpoint.
The vertical motion of the towpoint was varied over a frequency range of 0 to 2.8 radians per second with a constant amplitude of
1 foot.
In addition to basin tests, it was deemed advisable to also
make a dynamic calibration of the gyro and pendulum units. This
was accomplished by means of the swing shown in Figure 3. Both
indicators were located in the box, as shown, and subjected to known motions controlled by the variable speed motor and measured
accurately with the angular potentiometer. Comparison was made
between the angle measured by the potentiometer and that indicated
Instrument Manufacturer Type Cost (Approx.) Power Accuracy (stati Principle Pendulum Edcliff Duarte, California $160 DC Battery TABLE 1
Comparison of Gyro and Pendulum
0.5 deg
Pendulum depends on gravity to give the vertical reference. The pendulum mass supported by instru-ment type ball bearing, actuates a potentio-meter producing an output linear with rotation. Liquid in case provides fluid damping. 3 Gyro Minneapolis-Honeywell Minneapolis, Minnesota Vertical, Self-Erecting JG-10414A $1,085 AC-DC 115v.,
400
cps single phase 4 0.15 deg Rapid spinning of gyro rotor provides stable reference. Long term drift com-pensated by mercury switches which actu-ates erection motors to maintain vertical alignment of gyro spin axis. Conductors Req'd I 3 Model Ident. 5-5-1 5 1/8" x 57/8" x 7 3/4"
5.0 lb Size 2.0" dia. x 1.1" Weight 0.75 lbBASIN TESTS
The results of tests with the towed body are given in Table 2.
The sign convention is plus for nose-up attitude and minus for
nose-down.
TABLE 2
Comparison of Recorded Body Pitch from Gyro and
Pendulum Indicator
DISCUSSION OF RESULTS
Carriage Speed in Knots
0
+.2
1+4.2,- 4.4
-6.0,+ 8.6
L _+3.80- 6.2
+5.2,- 5.0
-(.3,+23.0
Figures 4a thru Lld show sample records of the simultaneous outputs from the pendulum and gyro for the various test conditions.
Analysis of these records shows that the pendulum output is quite
irregular,
particularly
at the higher speeds. Furthermore, thepitch
recorded by the pendulum is not in phaseIrith
the gyro output.At 10 knots and the higher frequencies this phase shift
iP about
180 degrees.
Figure 5 shows 9 compavison of the absolute values of
max-imum pitch amE:itude
(10mald)
measured by the gyro and ,Jer!':,11:m for various input frequencies (w). At low speeds and frequencies the pitch measured by the pendulum is within 2 or 3 degrees of thtmeasured by the gyro. At the higher speeds and frequencies, howe,'e,,
Input Frequency in radians per second V1 = 2.5 1
V2 = 5.0
v3 - 7.5
col
=Gyro
Pend
+2.0
+1.5
+1.0
a)2= 2.0
Gyro +7.0,-11.2 Pend +7.2,-9.8
+6.6,-
(.2+6.o,- 8.0
+5.0,- 6.0
4-4.2,- 7.0
w, = 2.4
Gyro
+8.4,- 8.2
Pend+8.4,- 7.3
+6.0,- 9.2 +5.4,-10.1 +5.0,- 6.1+8.o,- 7.0
a
- 2.8
Gyro+13.2,-14.8
Pend
+13.0,-13.0
+11.0,-10.G+7.5,-10-O+7.o,- 6.8
-8.2,+15.8
-.3
0
the pendulum shows a pitch amplitude as much as 4.5 times higher
than that of the gyro. It should also be noted that, in the plot
of 10,,,x1 versus w, the variation of pitch response with frequency
shown by the pendulum is entirely different from that shown by the
gyro.
DYNAMIC CALIBRATION
The results of the dynamic calibration performed under known angular inputs in the laboratory are given in Figure 6a thru 6d
inclusive. These records show that the gyro gives an accurate measure of pitch both in amplitude and phase. All gyro readings
were within ±i-degree of that of the angular potentiometer with apparently no phase difference whereas the pendulum record shows amplitude errors of as much as 17 degrees and phase differences as
great as 180 degrees.
To gain some insight into the effect of acceleration on the gyro and pendulum, the horizontal component was computed for the
conditions of dynamic calibration. In this case, the horizontal
acceleration is due entirely to the angular motion of the box in
which the gyro and pendulum were mounted. In the following sketch (See Figure 3 also)
P is the position of the transducer, P(i,)
is the length of the swinging arm, 5P
e is the angular displacement of the arm at any time t, is the horizontal displacement of P at any time t,
and co is the circular frequency of the oscillating input.
Taking fixed axes and C as shown, the horizontal displacement of the point P with respect to the origin at 0 is given by
= 2 sin e
The second derivative with respect to time is then
d2 = (g cos 8 - 62 sin (9) [1]
dt2
For a harmonic input oscillation, the angular displacement is given;
by
= 00 sin cot
where
ea
is the maximum angular travel of the swinging arm.Substituting; the horizontal acceleration of the point P
is given by:
= -2060 [sin cot cos (9 + 60 cos2wt sin (3]
(2n-1)T
The maximum acceleration occurs at
e = eo
or at time t4
where T is the period and n = 1, 2,
3,
- - -
Also, 27the circular frequency by definition is w
= T.
Substituting, weobtain for the absolute value of the maximum horizontal acceleration:
6
-Imaxl = 1.00 w2
I"41axl = Lw29 cos
In this particular case, = 6 ft. and ao = 9.7 degrees - .169 radians whence:
win rad/sec
k in ft/sec2
From this relationship, the values of gmaxl were computed for each
emax
frequency w and plotted against values of for the gyro and Go
pendulum.
This comparison is given in Figure 7. As can be seen from the
sample calibration records given in Figure 6a thru Od, it was
difficult to determine a value of 9ma, for the pendulum. The value selected for each w was taken from an average of the recorded peaks.
The significance of such a value is, however, subject to considerable
question. Consequently, Figure 7 serves only to qualitatively il-lustrate the effect of frequency and horizontal acceleration on the pendulum error. It also clearly shows that there is no such effect
on the gyro over the range of values considered.
To further illustrate the effect of body acceleration on the
accuracy of pendulum indicators, an analysis was made for the case
of the towed body subjected to motions in the vertical plane. This analysis is presented in Appendix B.
CONCLUSIONS AND RECOMMENDATIONS
The results of this comparison between a gyro and pendulum indicator clearly demonstrate the danger in using pendulums to
measure angles under dynamic conditions. The range of input motions
applied to the towed vehicle used in making this comparison is typical of motions which could occur in towing such bodies from a
vessel operating in a seaway. This appears to preclude the use of pendulums for other than static measurement of angular attitude.
It is recommended, therefore, that future evaluations of cable-towed bodies such as the variable-depth-sonar be based on the use of
ACKNOWLEDGMENTS
The authors are indebted to Messrs. M. Graybill, J. Leahy and E. Frillman of the Applied Instrumentation Branch for their generous assistance in providing the instrumentation necessary
to make the measurements described in this report.
APPENDIX A. - THE DAVID TAYLOR MODEL BASIN
HEAVING TOWPOINT FACILITY
General Description and Purpose
The "heaving towpoint" is a device by means of which systematically varied vertical displacements can be applied
to the end of a cable from which a submerged body is towed.
This device is design to be attached to TMB Carriage 2 near
the 4, of the basin. This carriage has a top speed of about
17 knots. The purpose of this towpoint is to provide a means
of simulating the vertical motions of a real towing platform
such as a ship. The stability characteristics of towed bodies such as the variable depth sonar can thus be predicted more accurately from model tests.
Design Specifications
The tow point oscillates along an axis which is normal to a plane tangent to the earth's surface,
i.e., a line usually referred to as the vertical axis The total vertical displacement can be varied from 0
to 10 feet and the center of oscillation can be positioned
anywhere along the 10-foot length. It should be noted,
however, that the maximum amplitude is realized only when the center of oscillation is in the center of
the 10-foot length.
Either a single step function or a continuous
sinusoidal motion can be produced.
For the sinusoidal motion the double amplitude is variable from 0 to 10 feet and the period is
variable from about 3 seconds to infinity.
The maximum design loading on the towpoint is
1000 pounds horizontal (drag) force, 1000 pounds
vertical force (including dead load) and 500
pounds side force.
Maximum Permissible Model Size
The maximum size of model to be used on this facility will
be determined by the maximum loads specified above. These
loads are a function of several factors; namely, the shape of the body, the length of cable, the mass of the body, the accelerations which are to be imposed, and the maximum towing
speed. It is seen, therefore, that the maximum model size can
We can, however, obtain some idea of the limits imposed on the
simulation
of
full-scale parameters. This can bedone
sincewe know that the laws of dynamic scaling will be followed in
determining full-scale values based on model tests. In other
words, when we model a dynamic towing test we preserve the
full-scale value of the ratio V__, where V is the speed of
igL
advance, L is a characteristic length dimension, and g is the
acceleration
of
gravity. On this basis, thefollowing
scale relations are obtained:For a model scale
of X,
Full-scale length X -Model-scale length Full-scale time Model time -x2 Full-scale velocity Model velocity x2
Model linear accelerations = Full-scale linear accelerations
Model angular accelerations - Full-scale angular accelerationsq
Full-scale forces Model forces
-X3
As a result, the maximum full-scale loads which can be duplicated
are:
Full-scale horizontal (drag) force = 1000 X3 pounds
Full-scale vertical force = 1000 X3 pounds
Full-scale side force = 500 X3 pounds
Within the allowable loads then, the towpoint can produce a
sinusoidal motion
with a maximum full-scale doublq-amplitudeof 10
X feet and minimum full-scale period of 3 A.-2 seconds,with linear accelerations equal for both model and full-scale. 1
APPENDIX B - ANALYSIS OF THE EFFECTS OF TOWED BODY MOTION ON A PENDULUM ANGLE INDICATOR
In the comparison of the gyro and pendulum for the laboratory
calibration, the origin 0 was fixed in space. As a result, the
acceleration of the point P was due only to rotation about a fixed
point.
In order to interpret the results of the gyro and the pendulum mounted in the body when undergoing oscillations under the sinusoidal
input from the heaving towpoint, it is necessary to consider an additional horizontal acceleration component at the point P in
addition to that at the origin,O. Here, the point 0 is in a movIng
axes system with freedom in translation as well as rotation. Using axes fixed to the body, we take Y positive toward the nose of the body and z positive downwards perpendicular to x, as in the follnwL1g
sketch:
We may define:
b = distance of point P from origin 0
e and C coordinate axes fixed in space
The instantaneous velocity of the origin may then be written
= lu + 'RIAT
where u and w are the velocity components along the respective x
and z body axes assuming two dimensional motion in the vertical
plane. The acceleration vector at 0 may be obtained by taking
the substantial derivative of the velocity vector,
D
yo = do
(E) x Vo)Dt dt
where the vector cross product of the angular velocity vector aS
and the linear velocity vector Vo is given by
assuming only rotational motion about the transverse axis.
The acceleration of the towpoint with q = 0 can then be written as
D
vo = I
+ ew) + Tc (W - eu)
Dt
This must be combined vectorially with the linear component due to the angular acceleration of the point P rotating with respect to the origin 0 to get the total acceleration that the
transducer experiences.
The point P where the transducer is located can be referenced
to the origin by [2] 12
7
FD- x -70 0 q 0 u 0 w-xp = b sin 0 = b sin (5-0)
Zp = b cos p = b cos (b-e)
The velocity and acceleration of point P which is fixed to
the body relative to the origin 0 can be obtained by taking the
first and second derivatives with respect to time.
= - IDE5
COS (5-e)
ip = be sin (5-9)
Kp = - bg cos (5-0) - bP sin (5-9)
2P -- be sin (5-e) - be2 cos (5-e)
[3c]
Combining the corresponding components from Equations [2] and
[3c], the acceleration of the point P may be given by its
longi-tudinal and vertical components along body axes as
ax 1.1 + we
-
be2 sin (s-(9) - bg cos (54)[4]
az = *t;T
- ue -
be2 cos (5-e) + b.() sin (5-e)Since the pendulum transducer works on the gravity principle,
we must refer the accelerations of the point P measured along body
axes to that of space axes. Using and C for the space axes as
shown in the preceding sketch, the relationship between the
accelerations along body and space axes are given by
= ax cos e + az sin
[5]
C = az cos 6 - ax sin
Considering only the horizontal space acceleration, the
resolution of can be obtained by substituting ax and az from
Equation [4] respectively in Eluation [5]. The resultant equation
for the horizontal space acceleration of the point P can be stated
as
-= U
This can be demonstrated by sliding the pendulum transducer
back and forth on a flat table to simulate pure surging. By such
means the mass in the pendulum could easily be made to move giving
erroneous readings.
However, in finally discussing the sum error of the pendulum transducer, the differential equations of motion of the mass in the transducer must be investigated assuming knowledge of the inertia, damping and spring constants in addition to the external
forces induced by the kinematics of the point P. The frequency
response of this subsidiary system of pendulum mass with fluid damping and friction can then be studied to show the errors in
magnitude and phasing of the output. The discussions previously mentioned are restricted to the kinematics of a point P where the
transducer is located to show the nature of the input to this
subsidiary compound pendulum system.
= U COS 0 * sin 9 + 9 (w cos - u sin 9)
- 13E52 sin
(e+p) - h5
cos(e+p)
[6]
where (9113) = 5
To determine the magnitude of the horizontal space
accelera-tion, therefore, it is necessary to obtain u and w. Unfortunately, these were not obtained for the towing tests previously described.
By comparing Equations
[6]
to [1], however, it can readily be seenthat now there are more components in the expression for the
horizontal space acceleration. There is possibility of a greater
or less error in pitch measurement using the pendulum when there
is surging and heaving of the body. For the range of conditions
tested, the pendulum gave a pitch reading about 4.5 times that of
the gyro reading in the worst case. This can be attributed to the u component arising from increased surging at the higher speeds.
It is also evident from Eluation
[6]
that the location of the transducer characterized by the arm b and the angle p has aninfluence on the magnitude of the acceleration. If the transducer
were to be located on the towpoint itself such that b = 0, the
latter two terms in [6] would drop out.
In addition, if there was no pitching of the fish for the
zero trim condition such that 9, 8, and 0 were zero, the horizontal space acceleration in Equation
[6]
reduces to one of pure surge,PSD 68620
6-5-57
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Responseof
Towed Body as Indicated
by Gyro and by Edcliff Pendulum,
(D. 0 Radians per Second
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Figure 5
Comparison of 1. aximum Pitch Amplitudes of Towed Body
as Measured by Gyro and Pendulum for Different Input
Frequencies
3.0
Legend:
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Ili
Edcliff Pendulum
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-2 3 6 7 9 2 1INITIAL DISTRIBUTION
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